In the following description of favorable exemplary embodiments of the present invention, identical or similar reference numerals are used for the similarly functioning elements represented in the different figures, a repeated description of these elements being dispensed with.
FIG. 1 shows a block diagram of a measuring system 100 including a device 105 according to one exemplary embodiment of the present invention. Measuring system 100 includes a measuring sensor 110, for example, in the form of a micromechanical acceleration sensor, which provides a measuring signal 115. This measuring signal 115 is often interfered with during the transmission to device 105 by parasitic couplings such as an electromagnetic coupling of an interference signal 120, however, so that during the ascertainment of the pure measuring signal 115 a part of interference signal 120 possibly mixed into the frequency range to be evaluated by device 105 is wrongly perceived as part of measuring signal 115 and, therefore, the measuring signal may not be precisely detected. This may have considerable disadvantages in some cases or may entirely prevent the use of ascertained measuring signal 115 for an intended purpose when measuring signal 115 is used, for example, for activating passenger protection systems in a vehicle.
In order to now enable the immunity to interference to be optimized when detecting measuring signal 115, according to the present invention, a sampling frequency generator 125 is utilized in device 105, which provides a first sampling frequency fA1 and a second sampling frequency fA2, which differs from the first sampling frequency. Measuring signal 115 interfered with by interference signal 120 is sampled by a first analog-to-digital converter 130 (ADC) using first sampling frequency fA1 in order to obtain a first sampled signal 135. In a second analog-to-digital converter 140, measuring signal 115 interfered with by interference signal 120 is sampled using second sampling frequency fA2 in order to obtain a second sampled signal 145.
FIG. 2 shows diagrams of signal components 200 of first sampled signal 135 and second sampled signal 145 for ascertaining useful signal 155. In the diagrams, the amplitudes of first sampled signal 135 and second sampled signal 145 are plotted against the frequency. The exemplary embodiment represented in FIG. 2 is based on a measuring signal 115 (an acceleration signal, for example, in this case) having a frequency of 0.5 kHz, an interference signal of an EMC interference having a frequency of 124 kHz, a first sampling frequency of fA1=125 kHz, and a second sampling frequency of fA2=128 kHz. In this case, it is apparent that a signal component 200 occurs in both first and second sampled signals 135 and 145 in frequency range 210 (from 0 to 2 kHz) to be evaluated by detection unit 150, at the frequency of 0.5 kHz of actual measuring signal 115, whereas, in first sampled signal 135, a first false signal (pseudosignal) 220 of frequency 1 kHz occurs as a result of a mixing effect 215 of interference signal 120 of frequency 124 kHz at first sampling frequency fA1, and, in second sampled signal 145, a second false signal (pseudosignal) 225 of frequency 4 kHz occurs as a result of a mixing effect 215 of interference signal 120 of frequency 124 kHz at second sampling frequency fA2. First and second false signal 220 and 225, respectively, are therefore interference signals, which are not to be taken into account in the detection of measuring signal 115 as useful signal 155. According to the even more detailed description below, useful signal 155 is now detected in a detection unit 150 as that part of measuring signal 115 superimposed by interference signal 120 that has the same frequency 228 within a tolerance range 227 (of 10%, for example).
It is particularly advantageous when the spacing between the two ADC frequencies fA1 and fA2 is greater than twice the maximum signal frequency to be processed. An interfering frequency located exactly between the two ADC frequencies fA1 and fA2 is therefore prevented from generating a “valid signal.”
In detection unit 150, it may now be detected, as a result of the evaluation of first sampled signal 135 and second sampled signal 145, that a signal component 200 is contained in each of these sampled signals, in the range of 0.5 kHz (allowing for a 10% deviation, for example, from this value of 0.5 kHz), whereas a further signal component is contained at a frequency of 1 kHz only in first sampled signal 135 in frequency range 210 to be evaluated, which therefore cannot originate from measuring signal 115. Component 225 in the second sampled signal, which has been down-converted from interference signal 120, is outside frequency range 210 to be evaluated, on the one hand, and does not have a corresponding counterpart in first sampled signal 135 and, therefore, may be detected as pseudosignal 225 generated as a result of mixing. Signal components 200 in first and second sampled signal 135 and 145, respectively, are therefore recognizable as signal components of a real signal, so that useful signal 155 may be output by detection unit 150 as corresponding signal components 200.
It is also possible that a frequency of interference signal 120 is identified on the basis of the knowledge of first and second sampling frequency fA1 and fA2, respectively, and first and second pseudosignal 220 and 225, respectively, so that this information may also be output, for example, by detection unit 150 and may be utilized for avoiding further interferences by interference signal 120 or for its compensation.
In other words, the approach presented here makes it possible to optimize the immunity to interference when evaluating signals using ADCs (analog-to-digital converters) or in the case of micromechanical sensors. Such pseudosignals 220 and 225 may be identified by using two ADC units 130 and 140, respectively, having different processing frequencies fA1 and fA2, respectively. Only real acceleration signals, of the type which result in signal components 200, yield identical values in the case of both ADCs 130 and 140. As a result of the comparison of the two ADC outputs or signals 135 and 145, real acceleration values 115 and pseudosignals 220 and 225, may be cleanly detected and processed.
In addition, an interference by a test signal 230 (for example, having a frequency of 0.75 kHz) could also be intentionally applied during regular operation and, therefore, both ADC units 130 and 140 could be checked for the correct functionality. This test signal 230 is then detected as intentionally generated test pseudosignal 230 and is then checked for correctness. Therefore, if a signal component is present in first sampled signal 135 and second sampled signal 145 at the frequency 0.75 kHz, it may be ensured that sampling units 130 and 140 function correctly.
Regular interferences on supply line 250 (see FIG. 1) may also be used as test signal 230, e.g., when a DC/DC or AC/DC converter operates at an appropriate frequency, electric filters 260 of supply voltage 250 may play less of a role, so that a certain portion of known interferences is on supply line 250 and may then be used as the test signal.
Using this special method, supply filter 260 which is used may even be simultaneously checked for correct function. Misassemblies/damaged components (keyword: FMEA) in the case of assembled circuit boards result in other filter characteristics and then generate a different test signal 230 or test result in sampled signals 135 and 145.
A further embodiment is the utilization (not shown in FIG. 1) of a second sensor element, which then runs together with the second ADC unit completely at other drive/processing frequencies.
The embodiment described with reference to FIG. 1 may be considered to be optimal in terms of cost, since, due to the high level of integration of the ASIC/silicon processes used today, an additional ADC unit does not result in notable additional costs (as compared to other optimization possibilities for reducing EMC interferences in ADCs).
FIG. 3 shows a flow chart of a method 300 for ascertaining a useful signal according to one exemplary embodiment of the present invention. Method 300 includes a step 310 of sampling a read-in sensor signal using a first sampling frequency in order to obtain a first sampling signal and sampling the sensor signal using a second sampling frequency in order to obtain a second sampling signal, the first and the second sampling frequency differing from one another. Furthermore, method 300 includes a step 320 of detecting the useful signal as signal components of the first and the second sampled signal, the signal components of the first and the second sampled signal having the same frequency within a tolerance range.
If an exemplary embodiment includes an “and/or” linkage between a first feature and a second feature, this is intended to be read that the exemplary embodiment according to one specific embodiment includes both the first feature and the second feature and, according to a further specific embodiment, includes either only the first feature or only the second feature.